Re: [RFC 5/5] mm, page_alloc: disable pcplists during page isolation

[David Hildenbrand]

On 09.09.20 13:36, Michal Hocko wrote:
> On Wed 09-09-20 12:48:54, Vlastimil Babka wrote:
>> Here's a version that will apply on top of next-20200908. The first 4 patches need no change.
>>
>> ----8<----
>> >From 8febc17272b8e8b378e2e5ea5e76b2616f029c5b Mon Sep 17 00:00:00 2001
>> From: Vlastimil Babka <vbabka@xxxxxxx>
>> Date: Mon, 7 Sep 2020 17:20:39 +0200
>> Subject: [PATCH] mm, page_alloc: disable pcplists during page isolation
>>
>> Page isolation can race with process freeing pages to pcplists in a way that
>> a page from isolated pageblock can end up on pcplist. This can be fixed by
>> repeated draining of pcplists, as done by patch "mm/memory_hotplug: drain
>> per-cpu pages again during memory offline" in [1].
>>
>> David and Michal would prefer that this race was closed in a way that callers
>> of page isolation don't need to care about drain. David suggested disabling
>> pcplists usage completely during page isolation, instead of repeatedly draining
>> them.
>>
>> To achieve this without adding special cases in alloc/free fastpath, we can use
>> the same 'trick' as boot pagesets - when pcp->high is 0, any pcplist addition
>> will be immediately flushed.
>>
>> The race can thus be closed by setting pcp->high to 0 and draining pcplists
>> once in start_isolate_page_range(). The draining will serialize after processes
>> that already disabled interrupts and read the old value of pcp->high in
>> free_unref_page_commit(), and processes that have not yet disabled interrupts,
>> will observe pcp->high == 0 when they are rescheduled, and skip pcplists.
>> This guarantees no stray pages on pcplists in zones where isolation happens.
>>
>> We can use the variable zone->nr_isolate_pageblock (protected by zone->lock)
>> to detect transitions from 0 to 1 (to change pcp->high to 0 and issue drain)
>> and from 1 to 0 (to restore original pcp->high and batch values cached in
>> struct zone). We have to avoid external updates to high and batch by taking
>> pcp_batch_high_lock. To allow multiple isolations in parallel, change this
>> lock from mutex to rwsem.
>>
>> For callers that pair start_isolate_page_range() with
>> undo_isolated_page_range() properly, this is transparent. Currently that's
>> alloc_contig_range(). __offline_pages() doesn't call undo_isolated_page_range()
>> in the succes case, so it has to be carful to handle restoring pcp->high and batch
>> and unlocking pcp_batch_high_lock.
> 
> I was hoping that it would be possible to have this completely hidden
> inside start_isolate_page_range code path. If we need some sort of
> disable_pcp_free/enable_pcp_free then it seems like a better fit to have
> an explicit API for that (the naming would be obviously different
> because we do not want to call out pcp free lists). I strongly suspect
> that only the memory hotplug really cares for this hard guanrantee.
> alloc_contig_range simply goes with EBUSY.

There will be different alloc_contig_range() demands in the future: try
fast (e.g., loads of small CMA allocations) vs. try hard (e.g.,
virtio-mem). We can add ways to specify that.

-- 
Thanks,

David / dhildenb